Silver-carboxylate/1,2-diazine compounds as silver sources in photothermographic and thermographic elements

ABSTRACT

Novel silver-carboxylate/1,2-diazine compounds having the formula: ##STR1## wherein: R 1  represents either an alkyl, aralkyl, cycloalkyl, or alkenyl group of up to 29 carbon atoms; or an aryl group of up to 14 carbon atoms; and 
     and R 2  represents either hydrogen, an alkyl group, a cycloalkyl group fused to the 1,2-diazine ring, or the atoms necessary to complete a 5- or 6-membered aromatic ring fused to the 1,2-diazine ring. 
     The novel silver-carboxylate/1,2-diazine compounds are coordination compounds of two silver-carboxylate molecules and two molecules comprising a 1,2-diazine nucleus. These compounds can serve as the silver source in a black-and-white thermographic or photothermographic element or as the oxidizing agent for a dye-based thermographic or photothermographic color-imaging construction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to novel coordination compounds of silver for usein black-and-white and color thermographic and photothermographicimaging elements and to thermographic and photothermographic elementsemploying these materials.

2. Background to the Art

Silver halide-containing photothermographic imaging materials (i.e.,heat-developable photographic elements) processed with heat, and withoutliquid development, have been known in the art for many years. Thesematerials, also known as "dry silver" compositions or emulsions,generally comprise a support having coated thereon: (1) a photosensitivematerial that generates elemental silver when irradiated; (2) anon-photosensitive, reducible silver source; and (3) a reducing agentfor the non-photosensitive reducible silver source; and (4) a binder.The photosensitive material is generally photographic silver halidewhich must be in catalytic proximity to the non-photosensitive,reducible silver source. Catalytic proximity requires an intimatephysical association of these two materials so that when silver specksor nuclei are generated by the irradiation or light exposure of thephotographic silver halide, those nuclei are able to catalyze thereduction of the reducible silver source. It has long been understoodthat elemental silver (Ag°) is a catalyst for the reduction of silverions, and the photosensitive photographic silver halide may be placedinto catalytic proximity with the non-photosensitive, reducible silversource in a number of different fashions, such as by partial metathasisof the reducible silver source with a halogen-containing source (see,for example, U.S. Pat. No. 3,457,075); coprecipitation of silver halideand reducible silver source material (see, for example, U.S. Pat. No.3,839,049); and other methods that intimately associate thephotosensitive photographic silver halide and the non-photosensitive,reducible silver source.

The non-photosensitive, reducible silver source is a material thatcontains silver ions. The preferred non-photosensitive, reducible silversource comprises silver salts of long chain aliphatic carboxylic acids,typically having from 10 to 30 carbon atoms. The silver salt of behenicacid or mixtures of acids of similar molecular weight are generallyused. Salts of other organic acids or other organic materials, such assilver imidazolates, have been proposed, and U.S. Pat. No. 4,260,677discloses the use of complexes of inorganic or organic silver salts asnon-photosensitive, reducible silver sources.

In both photographic and photothermographic emulsions, exposure of thephotographic silver halide to light produces small clusters of silveratoms (Ag°). The imagewise distribution of these clusters is known inthe art as a latent image. This latent image generally is not visible byordinary means and the photosensitive emulsion must be further processedin order to produce a visible image. The visible image is produced bythe reduction of silver ions, which are in catalytic proximity to silverhalide grains bearing the clusters of silver atoms, i,e, the latentimage. This produces a black-and-white image.

As the visible image is produced entirely by elemental silver (Ag°), onecannot readily decrease the amount of silver in the emulsion withoutreducing the maximum image density. However, reduction of the amount ofsilver is often desirable in order to reduce the cost of raw materialsused in the emulsion.

One method of attempting to increase the maximum image density inblack-and-white photographic and photothermographic emulsions withoutincreasing the amount of silver in the emulsion layer is byincorporating toning agents into the emulsion. Toning agents improve thecolor of the silver image of the photothermographic emulsions, asdescribed in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.

Another method of increasing the maximum image density of photographicand photothermographic emulsions without increasing the amount of silverin the emulsion layer is by incorporating dye-forming materials in theemulsion. For example, color images can be formed by incorporation ofleuco dyes into the emulsion. Leuco dyes are the reduced form of acolor-bearing dye. Upon imaging, the leuco dye is oxidized, and thecolor-bearing dye and a reduced silver image are simultaneously formedin the exposed region. In this way a dye enhanced silver image can beproduced, as shown, for example, in U.S. Pat. Nos. 3,531,286; 4,187,108;4,426,441; 4,374,921; and 4,460,681.

Multicolor photothermographic imaging articles typically comprise two ormore monocolor-forming emulsion layers (often each emulsion layercomprises a set of bilayers containing the color-forming reactants)maintained distinct from each other by barrier layers. The barrier layeroverlaying one photosensitive, photothermographic emulsion layertypically is insoluble in the solvent of the next photosensitive,photothermographic emulsion layer. Photothermographic articles having atleast 2 or 3 distinct color-forming emulsion layers are disclosed inU.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dyeimages and multicolor images with photographic color couplers and leucodyes are well known in the art as represented by U.S. Pat. Nos.4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; andResearch Disclosure, March 1989, item 29963.

Thermographic imaging constructions (i.e., heat-developable materials)processed with heat, and without liquid development, are widely known inthe imaging arts and rely on the use of heat to help produce an image.These elements generally comprise a support or substrate (such as paper,plastics, metals, glass, and the like) having coated thereon: (1) athermally-sensitive, reducible silver source; (2) a reducing agent forthe thermally-sensitive, reducible silver source; and (3) a binder.

In a typical thermographic construction, the image-forming layers arebased on silver salts of long chain fatty acids, such as silverbehenate. At elevated temperatures, silver behenate is reduced by areducing agent for silver ion such as methyl gallate, hydroquinone,substituted-hydroquinones, hindered phenols, catechol, pyrogallol,astorbit acid, ascorbic acid derivatives, and the like, whereby an imagecomprised of elemental silver is formed. When the reducing agent is amaterial that can be oxidized to form or release a dye, as, for example,a leuco dye, a colored image is formed.

Many times, the thermographic construction is brought into contact withthe thermal head of a thermographic recording apparatus, such as athermal printer, thermal facsimile, and the like. In such instances, ananti-stick layer is coated on top of the imaging layer to preventsticking of the thermographic construction to the thermal head of theapparatus utilized. The resulting thermographic construction is thenheated to an elevated temperature, typically in the range of about60°-225° C., resulting in the formation of an image.

Phthalazine (PHZ), (i.e., 4,5-benzo-1,2-diazine), is known as a tonerand is added to photothermographic formulations. There are no reports ofpolymeric silver-carboxylate/PHZ coordination compounds, however. Thecompound Ag(PHZ)₂ NO₃ is reported in T. Tsuda, S. Ohba, M. Takahashi andM. Ito, Acta Cryts. 1989, C45, 887. A similar compound, Ag(PHZ)NO₃,complex having a 1:1 Ag:PHZ ratio is described in J. J. Porter, J. L.Murray, and K. B. Takvorian J. Heterocyclic Chem. 1973, 10, 43.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, newsilver-carboxylate/1,2-diazine compounds have been discovered. Thesesilver-carboxylate/1,2-diazine compounds are coordination complexes oftwo silver-carboxylate molecules and two molecules comprising a1,2-diazine nucleus. These compounds, herein referred to as silvercarboxylate/1,2-diazine compounds, can serve as the silver source in ablack-and-white thermographic or photothermographic element or as theoxidizing agent for a dye-based thermographic or photothermographiccolor-imaging construction.

The silver-carboxylate/1,2-diazine compounds can be represented by thefollowing formula: ##STR2## wherein:

R¹ represents either an alkyl, aralkyl, cycloalkyl, or alkenyl group ofup to 29 carbon atoms; preferably of from 9 to 29 carbon atoms; and mostpreferably of from 14 to 27 carbon atoms; or an aryl group of up to 14carbon atoms, preferably of up to 10 carbon atoms;

and R² represents either hydrogen, an alkyl group, a cycloalkyl groupfused to the 1,2-diazine ring, or the atoms necessary to complete a 5-or 6-membered aromatic ring fused to the 1,2-diazine ring and preferablyrepresents the atoms necessary to complete a 5- or 6-membered aromaticring fused to the 1,2-diazine ring.

In another embodiment, the present invention provides heat-developable,photothermographic elements capable of providing stable, high densityimages of high resolution. These elements comprise a support bearing atleast one photosensitive, image-forming, photothermographic emulsionlayer comprising:

(a) a photosensitive silver halide;

(b) a non-photosensitive, reducible source of silver comprising asilver-carboxylate/1,2-diazine compound of formula (I);

(c) a reducing agent for the non-photosensitive, reducible source ofsilver; and

(d) a binder.

In still another embodiment, the present invention comprises athermographic construction comprising a substrate coated with an imaginglayer comprising:

(a) a non-photosensitive, reducible source of silver comprising asilver-carboxylate-1,2-diazine compound of formula (I);

(b) a reducing agent for the non-photosensitive, reducible source ofsilver; and

(c) a binder.

In both the inventive thermographic and photothermographicconstructions, the reducing agent for the non-photosensitive silversource may optionally comprise a compound capable of being oxidized toform a dye or to release a pre-formed dye. Preferably, the dye-formingmaterial is a leuco dye.

As is well understood in this technical area, a large degree ofsubstitution is not only tolerated, but is often advisable. As a meansof simplifying the discussion and recitation of certain terminology usedthroughout this application, the terms "group" and "moiety" are used todifferentiate between chemical species that allow for substitution orwhich may be substituted and those which do not so allow or may not beso substituted. Thus, when the term "group" is used to describe achemical substituent, the described chemical material includes the basicgroup and that group with conventional substitution. Where the term"moiety" is used to describe a chemical compound or substituent, only anunsubstituted chemical material is intended to be included. For example,the phrase "alkyl group" is intended to include not only pure open-chainand cyclic saturated hydrocarbon alkyl substituents, such as methyl,ethyl, propyl, t-butyl, cyclohexyl, adamantyl, octadecyl, and the like,but also alkyl substituents bearing further substituents known in theart, such as hydroxyl, alkoxy, vinyl, phenyl, halogen atoms (F, Cl, Br,and I), cyano, nitre, amine, carboxyl, etc. On the other hand, thephrase "alkyl moiety" is limited to the inclusion of only pureopen-chain and cyclic saturated hydrocarbon alkyl substituents, such asmethyl, ethyl, propyl, t-butyl, cyclohexyl, adamantyl, octadecyl, andthe like.

Other aspects, advantages, and benefits of the present invention areapparent from the detailed description, the examples, and the claims.

DETAILED DESCRIPTION OF THE INVENTION

A new class of silver compounds have been discovered which are useful asa unique source of the silver oxidant in black-and-white (B&W)thermographic and photothermographic materials. These are comprised ofcoordination compounds of silver-carboxylate/1,2-diazine compoundshaving the following formula: ##STR3##

In formula (I), R¹ is either an alkyl, aralkyl, cycloalkyl, or alkenylgroup of up to 29 carbon atoms, preferably of from 9 to 29 carbon atoms,and most preferably of from 14 to 27 carbon atoms; or an aryl group ofup to 14 carbon atoms, preferably of up to 10 carbon atoms.

In formula (I), R² represents either hydrogen, an alkyl group, acycloalkyl group fused to the 1,2-diazine ring, or the atoms necessaryto complete a 5- or 6-membered aromatic ring fused to the 1,2-diazinering.

The photosensitive element of the present invention comprises a supporthaving at least one photosensitive image-forming, photothermographicemulsion layer comprising:

(a) a photosensitive silver halide;

(b) a non-photosensitive, reducible source of silver comprising asilver-carboxylate/1,2-diazine compound of formula (I);

(c) a reducing agent for the non-photosensitive, reducible source ofsilver; and

(d) a binder.

In another embodiment, the present invention comprises a thermographicconstruction comprising a substrate coated with an imaging layercomprising:

(a) a non-photosensitive, reducible source of silver comprising asilver-carboxylate/1,2-diazine compound of formula (I);

(b) a reducing agent for the non-photosensitive, reducible source ofsilver; and

(c) a binder.

In both the inventive thermographic and photothermographicconstructions, the reducing agent for the non-photosensitive silversource may optionally comprise a compound capable of being oxidized toform a dye or to release a pre-formed dye. Preferably, the dye-formingmaterial is a leuco dye.

The silver-carboxylate/1,2-diazine coordination compounds of the presentinvention can be made according to methods of synthetic inorganicchemistry which will be apparent to those of ordinary skill in the art.The compounds are prepared by reaction of two molecules of a silver saltof an organic acid (i.e. a silver-carboxylate), particularly the silversalt of a long chain fatty carboxylic acid, wherein the chain typicallycontains 10 to 30, and preferably 15 to 28, carbon atoms, with twomolecules of a compound comprising a 1,2-diazine nucleus. Thepreparation of such compounds is also detailed later in this applicationin Example 1. The silver-carboxylate/1,2-diazine compounds of theinvention are believed to have formula (I) shown above. In formula (I),two molecules having a 1,2-diazine nucleus are coordinated to twomolecules of a silver carboxylate. As used in this application, thephrase "1,2-diazine nucleus" is meant to denote any molecule having a6-membered aromatic ring containing a --N═N--, (or its resonanceequivalent, ═N--N═) aromatic sp² moiety.

Non-limiting preferred examples of molecules having a 1,2-diazinenucleus useful in this invention are; 1,2-diazine (pyridazine);3,4-benzo-1,2-diazine (cinnoline); 4,5-benzo-1,2-diazine(phthalazine--PHZ); and 3,4,5,6-dibenzo-1,2-diazine (benzo[c]cinnoline).The structures of these compounds are shown below. Most preferred aremolecules having a 4,5-benzo-1,2-diazine (phthalazine--PHZ) nucleus.##STR4##

The structure of a crystal of Ag₂ (O₂ CCH₃)₂ (PHZ)₂, determined by X-raydiffraction, indicates it to be made up of individual molecules of Ag₂(O₂ CCH₃)₂ (PHZ)₂ stacked one above the other. The distance betweenindividual atoms indicates each silver to be 5-coordinate, bycoordination to an oxygen of a carboxylate of an adjacent Ag₂ (O₂ CCH₃)₂(PHZ)₂ molecule in addition to the carboxylate to which it is attached.In the Ag₂ (O₂ CCH₃)₂ (PHZ)₂ molecule itself, there are two molecules ofwater present in the unit cell. It is assumed that preferred moleculesof the present invention derived from longer chain carboxylic acids havea similar crystal structure. However, they may lack water moleculeswithin the crystal.

The photothermographic elements of this invention may be used to prepareblack-and-white, monochrome, or full color images. Thephotothermographic element of this invention can be used, for example,in conventional black-and-white or color photothermography, inelectronically-generated black-and-white or color hardcopy recording, inthe graphic arts area, and in digital color proofing. The element ofthis invention provides high photographic speed, provides stronglyabsorbing black-and-white or color images, and provides a dry and rapidprocess.

The thermographic elements of this invention can also be used to prepareto prepare black-and-white or monochrome images.

The Photosensitive Silver Halide

As noted above, when used in a photothermographic construction, thepresent invention comprises a photosensitive silver halide. Thephotosensitive silver halide can be any photosensitive silver halide,such as silver bromide, silver iodide, silver chloride, silverbromoiodide, silver chloro-bromoiodide, silver chlorobromide, silverchloroiodide, etc. The photosensitive silver halide can be added to theemulsion layer in any fashion so long as it is placed in catalyticproximity to the organic silver compound which serves as a source ofreducible silver.

The light sensitive silver halide used in the present invention can beemployed in a range of 0.005 mole to 0.5 mole and, preferably, from 0.01mole to 0.15, mole per mole of silver salt.

The silver halide used in the present invention may be employed withoutmodification. However, it can be chemically and spectrally sensitized ina manner similar to that used to sensitize conventional wet processsilver halide or state-of-the-art heat-developable photographicconstructions. For example, it may be chemically sensitized with achemical sensitizing agent such as a compound containing sulfur,selenium or tellurium etc., or a compound containing gold, platinum,palladium, ruthenium, rhodium or iridium, etc., a reducing agent such asa tin halide, etc., or a combination thereof. The details of theseprocedures are described in T. H. James, The Theory of the PhotographicProcess, Fourth Edition, Chapter 5, pages 149 to 169. Suitable chemicalsensitization procedures are also described in Shepard, U.S. Pat. No.1,623,499; Waller, U.S. Pat. No. 2,399,083; McVeigh, U.S. Pat. No.3,297,447; and Dunn, U.S. Pat. No. 3,297,446.

The photosensitive silver halides may be spectrally sensitized withvarious known dyes that spectrally sensitize silver halide. Non-limitingexamples of sensitizing dyes that can be employed include cyanine dyes,merocyanine dyes, complex cyanine dyes, complex merocyanine dyes,holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxanoldyes. Of these dyes, cyanine dyes, merocyanine dyes, and complexmerocyanine dyes are particularly useful.

An appropriate amount of sensitizing dye added is generally in the rangeof from about 10⁻¹⁰ to 10⁻¹ mole, and preferably from about 10⁻⁸ to 10⁻³moles, per mole of silver halide.

The Non-Photosensitive Reducible Silver Source Material

When used in photothermographic and thermographic constructions thepresent invention comprises a non-photosensitive reducible silver sourcematerial.

In addition to the a silver-carboxylate/1,2-diazine coordinationcompound, auxiliary non-photosensitive organic silver salts that can beused in the present invention are silver salts which are comparativelystable to light and which form a silver image by reacting with areducing agent.

The auxiliary non-photosensitive, reducible silver source can be anymaterial that contains a source of reducible silver ions. Silver saltsof organic acids, particularly silver salts of long chain fattycarboxylic acids, are preferred. The chains typically contain 10 to 30,preferably 15 to 28, carbon atoms.

When used in photothermographic elements, the silver halide and theorganic silver salt that form a starting point of development should bein reactive association (i.e., in the same layer, in adjacent layers, orlayers separated from each other by an intermediate layer having athickness of less than 1 micron). It is preferred that the silver halideand the organic silver salt are present in the same layer.

Non-limiting examples of such auxiliary silver salts include silversalts of organic compounds having a carboxyl group. Preferred examplesthereof include a silver salt of an aliphatic carboxylic acid and asilver salt of an aromatic carboxylic acid. Preferred examples of thesilver salts of aliphatic carboxylic acids include silver behenate,silver stearate, silver oleate, silver laureate, silver caprate, silvermyristate, silver palmirate, silver maleate, silver fumarate, silvertartarate, silver furoate, silver linoleate, silver butyrate and silvercamphorate, mixtures thereof, etc. Silver salts which are substitutablewith a halogen atom or a hydroxyl group can also be effectively used.Preferred examples of the silver salts of aromatic carboxylic acid andother carboxyl group-containing compounds include silver benzoate, asilver substituted benzoate such as silver 3,5-dihydroxybenzoate, silvero-methylbenzoate, silver m-methylbenzoate, silver p-methylbenzoate,silver 2,4-dichlorobenzoate, silver acetamidobenzoate, silverp-phenylbenzoate, etc., silver gallate, silver tannate, silverphthalate, silver terephthalate, silver salicylate, silverphenylacetate, silver pyromellilate, a silver salt of3-carboxymethyl-4-methyl-4-thiazoline-2-thione or the like as describedin U.S. Pat. No. 3,785,830, and silver salt of an aliphatic carboxylicacid containing a thioether group as described in U.S. Pat. No.3,330,663.

Auxiliary silver salts of compounds containing mercapto or thione groupsand derivatives thereof can be used. Preferred examples of thesecompounds include a silver salt of 3-mercapto-4-phenyl-1,2,4-triazole, asilver salt of 2-mercaptobenzimidazole, a silver salt of2-mercapto-5-aminothiadiazole, a silver salt of2-(2-ethylglycolamido)benzothiazole, a silver salt of thioglycolic acidsuch as a silver salt of a S-alkylthioglycolic acid (wherein the alkylgroup has from 12 to 22 carbon atoms) as described in Japanese PatentApplication No. 28221/73, a silver salt of a dithiocarboxylic acid suchas a silver salt of dithioacetic acid, a silver salt of thioamide, asilver salt of 5-carboxylic-1-methyl-2-phenyl-4-thiopyridine, a silversalt of mercaptotriazine, a silver salt of 2-mercaptobenzoxazole, asilver salt as described in U.S. Pat. No. 4,123,274, for example, asilver salt of 1,2,4-mercaptothiazole derivative such as a silver saltof 3-amino-5-benzylthio-1,2,4-thiazole, a silver salt of a thionecompound such as a silver salt of 3-(2-carboxyethyl)-4-methyl-4-thiazoline-2-thione as disclosed in U.S. Pat. No. 3,201,678.

Furthermore, an auxiliary silver salt of a compound containing an iminogroup can be used. Preferred examples of these compounds include asilver salt of benzothiazole and a derivative thereof as described inJapanese Patent Application Nos. 30270/69 and 18146/70, for example, asilver salt of benzothiazole such as silver salt of methylbenzotriazole,etc., a silver salt of a halogen-substituted benzotriazole, such as asilver salt of 5-chlorobenzotriazole, etc., a silver salt of1,2,4-triazole, of 1H-tetrazole as described in U.S. Pat. No. 4,220,709,a silver salt of imidazole and an imidazole derivative, and the like.

Additional auxiliary silver salts, silver half soaps, of which anequimolar blend of silver behenate and behenic acid, prepared byprecipitation from aqueous solution of the sodium salt of commercialbehenic acid and analyzing about 14 percent silver, represents apreferred example. Transparent sheet constructions made on transparentfilm backing require a transparent coating .and, for this purpose thesilver behenate full soap, containing not more than about 4 or 5 percentof free behenic acid and analyzing at about 25 percent silver may beused.

The method used for making silver soap dispersions is well known in theart and is disclosed in Research Disclosure April 1983 (22812), ResearchDisclosure October 1983 (23419) and U.S. Pat. No. 3,985,565.

The silver halide may be pre-formed and mixed with thesilver-carboxylate/1,2-diazine compound and any auxiliary organic silversalt in a binder to prepare a coating solution. It is also effective toblend the silver halide, the a silver-carboxylate/1,2-diazinecoordination compound, and any auxiliary organic silver salt in a ballmill for a long period of time. Materials of this type are oftenreferred to as "pre-formed emulsions." It is also effective to use an insitu process which comprises adding a halogen-containing compound to thesilver-carboxylate/1,2-diazine coordination compound and any auxiliaryorganic silver salt to partially convert the silver of thesilver-carboxylate/1,2-diazine coordination compound and any organicsilver salt to silver halide.

Methods of preparing these silver halide and auxiliary organic silversalts and manners of blending them are described in Research Disclosure,item No. 17029, Japanese patent applications No. 32928/75 and 42529/76,U.S. Pat. No. 3,700,458, and Japanese patent applications Nos. 13224/74and 17216/75.

Pre-formed silver halide emulsions in the construction of this inventioncan be unwashed or washed to remove soluble salts. In the latter casethe soluble salts can be removed by chill-setting and leaching or theemulsion can be coagulation washed, e.g., by the procedures described inHewitson, et al., U.S. Pat. No. 2,618,556; Yutzy et al., U.S. Pat. No.2,614;928; Yackel, U.S. Pat. No. 2,565,418; Hart et al., U.S. Pat. No.3,241,969; and Waller et al., U.S. Pat. No. 2,489,341. The silver halidegrains may have any crystalline habit including, but not limited to,cubic, tetrahedral, orthorhombic, tabular, laminar, platelet, etc.

The silver halide and the non-photosensitive reducible silver sourcematerial that form a starting point of development should be in reactiveassociation. By "reactive association" is meant that they should be inthe same layer, in adjacent layers, or in layers separated from eachother by an intermediate layer having a thickness of less than 1micrometer (1 μm). It is preferred that the silver halide and thenon-photosensitive reducible silver source material be present in thesame layer.

Photothermographic emulsions containing pre-formed silver halide inaccordance with this invention can be sensitized with chemicalsensitizers, or with spectral sensitizers as described above.

The total amount of non-photosensitive, reducible source of silver(silver-carboxylate/1,2-diazine coordination compound .and optionally,auxiliary organic silver salt compounds) is preferably present in anamount of from 15 to 70 percent by weight of the emulsion layer. It ismore preferably present at a level of 30 to 55 percent by weight of theemulsion layer.

When used in photothermographic elements of the present invention, theauxiliary organic silver salt is a silver salt which is comparativelystable to light, but forms a silver image when heated to 80° C. orhigher in the presence of an exposed photocatalyst (such as silverhalide) and a reducing agent.

When used in thermographic elements of the present invention, theauxiliary organic silver salt is a silver salt which is comparativelystable to light, but forms a silver image when heated to 80° C. orhigher in the presence of a reducing agent.

The Reducing Agent for the Non-Photosensitive Reducible Silver Source

When used in black-and-white photothermographic and thermographicconstructions the present invention comprises a reducing agent for thenon-photosensitive reducible silver source material.

The reducing agent for the organic silver salt may be any material,preferably organic material, that can reduce silver ion to metallicsilver. Conventional photographic developers such as methyl gallate,hydroquinone, substituted-hydroquinones, hindered phenols, catechol,pyrogallol, astorbit acid, astorbit acid derivatives, leuco dyes, etc.Hindered phenol reducing agents are preferred.

A wide range of reducing agents has been disclosed in dry silver systemsincluding amidoximes such as phenylamidoxime, 2-thienylamidoxime andp-phenoxyphenylamidoxime, azines (e.g.,4-hydroxy-3,5-dimethoxybenzaldehydeazine); a combination of aliphaticcarboxylic acid aryl hydrazides and astorbit acid, such as2,2'-bis(hydroxymethyl)propionylbetaphenyl hydrazide in combination withascorbic acid; a combination of polyhydroxybenzene and hydroxylamine, areductone and/or a hydrazine, e.g., a combination of hydroquinone andbis(ethoxyethyl)hydroxylamine, piperidinohexose reductone orformyl-4-methylphenylhydrazine, hydroxamic acids such asphenylhydroxamic acid, p-hydroxyphenylhydroxamic acid, ando-alaninehydroxamic acid; a combination of azines andsulfonamidophenols, e.g., phenothiazine and2,6-dichloro-4-benzenesulfonamidophenol; α-cyanophenylacetic acidderivatives such as ethyl α-cyano-2-methylphenylacetate, ethylα-cyano-phenylacetate; bis-o-naphthols as illustrated by2,2'-dihydroxyl-1-binaphthyl,6,6'-dibromo-2,2'-dihydroxy-1,1'-binaphthyl, andbis(2-hydroxy-1-naphthyl)methane; a combination of bis-o-naphthol and a1,3-dihydroxybenzene derivative, (e.g., 2,4-dihydroxybenzophenone or2,4-dihydroxyacetophenone); 5-pyrazolones such as3-methyl-1-phenyl-5-pyrazolone; reductones as illustrated bydimethylaminohexose reductone, anhydrodihydroaminohexose reductone, andanhydrodihydro-piperidone-hexose reductone; sulfamidophenol reducingagents such as 2,6-dichloro-4-benzenesulfonamidophenol, andp-benzenesulfonamidophenol; 2-phenylindane-1,3 -dione and the like;chromans such as 2,2-dimethyl-7-t-butyl-6-hydroxychroman;1,4-dihydropyridines such as2,6-dimethoxy-3,5-dicarbethoxy-1,4-dihydropyridine; bisphenols, e.g.,bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane;2,2-bis(4hydroxy-3-methylphenyl)propane;4,4-ethylidene-bis(2-t-butyl-6-methylphenol); and2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane; ascorbic acid derivatives,e.g., 1-ascorbylpaimitate, ascorbyistearate and unsaturated aldehydesand ketones; 3-pyrazolidones; and certain indane-1,3-diones.

The reducing agent should be present as 1 to 10% by weight of theimaging layer. In multilayer constructions, if the reducing agent isadded to a layer other than an emulsion layer, slightly higherproportions, of from about 2 to 15 wt %, tend to be more desirable.

The Optional Dye-Forming or Dye-Releasing Material

As noted above, the reducing agent for the reducible source of silvermay be a compound that can be oxidized directly or indirectly to form adye or to release a pre-formed dye.

The dye-forming or releasing material may be any colorless or lightlycolored compound that can be oxidized to a colored form, when heated,preferably to a temperature of from about 80° C. to about 250° C. (176°F. to 482° F.) for a duration of from about 0.5 to about 300 seconds.When used with a dye- or image-receiving layer, the dye can diffusethrough emulsion layers and interlayers into the image-receiving layerof the element of the invention.

Leuco dyes are one class of dye-releasing material that form a dye uponoxidation. Any leuco dye capable of being oxidized by silver ion to forma visible image can be used in the present invention. Leuco dyes thatare both pH sensitive and oxidizable can be used, but are not preferred.Leuco dyes that are sensitive only to changes in pH are not includedwithin scope of dyes useful in this invention because they are notoxidizable to a colored form.

As used herein, the term "change in color" includes: (1) a change froman uncolored or lightly colored state (optical density less than 0.2) toa colored state (an increase in optical density of at least 0.2 units);and (2) a substantial change in hue.

Representative classes of leuco dyes that are suitable for use in thepresent invention include, but are not limited to, bisphenol andbisnaphthol leuco dyes, phenolic leuco dyes, indoaniline leuco dyes,imidazole leuco dyes, azine leuco dyes, oxazine leuco dyes, diazineleuco dyes, thiazine leuco dyes, and triarylmethane leuco dyes.Preferred classes of dyes are described in U.S. Pat. Nos. 4,460,681 and4,594,307.

One class of leuco dyes useful in this invention are those derived fromimidazole dyes. Imidazole leuco dyes are described in U.S. Pat. No.3,985,565.

Another class of leuco dyes useful in this invention are those derivedfrom so-called "chromogenic dyes." These dyes are prepared by oxidalivecoupling of a p-phenylenediamine with a phenolic or anilinic compound.Leuco dyes of this class are described in U.S. Pat. No. 4,594,307. Leucochromogenic dyes having short chain carbamoyl protecting groups aredescribed in copending application U.S. Ser. No. 07/939,093,incorporated herein by reference.

A third class of dyes useful in this invention are "aldazine" and"ketazine" dyes. Dyes of this type are described in U.S. Pat. Nos.4,587,211 and 4,795,697.

Another class of leuco dyes are reduced forms of dyes having a diazine,oxazine, or thiazine nucleus. Leuco dyes of this type can be prepared byreduction and acylation of the color-bearing dye form. Methods ofpreparing leuco dyes of this type are described in Japanese Patent No.52-89131 and U.S. Pat. Nos. 2,784,186; 4,439,280; 4,563,415; 4,570,171;4,622,395; and 4,647,525.

Another class of dye-releasing materials that form a dye upon oxidationare known as preformed-dye-release (PDR) or redox-dye-release (RDR)materials. In these materials, the reducing agent for the organic silvercompound releases a pre-formed dye upon oxidation. Examples of thesematerials are disclosed in Swain, U.S. Pat. No. 4,981,775.

Also useful are neutral, phenolic leuco dyes such as2-(3,5-di-t-butyl-4-hydroxyphenyl)-4,5-diphenylimi dazole, orhis(3,5-di-t-butyl-4-hydroxy-phenyl)phenylmethane. Other phenolic leucodyes useful in practice of the present invention are disclosed in U.S.Pat. Nos. 4,374,921; 4,460,681; 4,594,307; and 4,782,010.

Other leuco dyes may be used in imaging layers as well, for example,benzylidene leuco compounds cited in U.S. Pat. No. 4,923,792. Thereduced form of the dyes should absorb less strongly in the visibleregion of the electromagnetic spectrum and be oxidized by silver ionsback to the original colored form of the dye. Benzylidene dyes haveextremely sharp spectral characteristics giving high color purity of lowgray level. The dyes have large extinction coefficients, typically onthe order of 10⁴ to 10⁵ mole-cm liter⁻¹, and possess good compatibilityand heat stability. The dyes are readily synthesized and the reducedleuco forms of the compounds are very stable. Leuco dyes such as thosedisclosed in U.S. Pat. Nos. 3,442,224; 4,021,250; 4,022,617; and4,368,247 are also useful in the present invention.

The dyes formed from the leuco dye in the various color-forming layersshould, of course, be different. A difference of at least 60 nm inreflective maximum absorbance is preferred. More preferably, theabsorbance maximum of dyes formed will differ by at least 80-100 nm.When three dyes are to be formed, two should preferably differ by atleast these minimums, and the third should preferably differ from atleast one of the other dyes by at least 150 nm, and more preferably, byat least 200 nm. Any leuco dye capable of being oxidized by silver ionto form a visible dye is useful in the present invention as previouslynoted.

The dyes generated by the leuco compounds employed in the elements ofthe present invention are known and are disclosed, for example, in TheColor Index; The Society of Dyes and Colorists: Yorkshire, England,1971; Vol. 4, p. 4437; and Venkataraman, K. The Chemistry of SyntheticDyes; Academic Press: New York, 1952; Vol. 2, p. 1206; and U.S. Pat. No.4,478,927.

Leuco dye compounds may readily be synthesized by techniques known inthe art. Suitable methods are disclosed, for example, in: F. X. Smith etal. Tetrahedron Lett. 1983, 24(45), 4951-4954; X. Huang., L. Xe, Synth.Commun. 1986, 16(13) 1701-1707; H. Zimmer et al. J. Org. Chem. 1960, 25,1234-5; M. Sekiya et al. Chem. Pharm. Bull. 1972, 20(2),343; and T.Sohda et al. Chem. Pharm. Bull. 1983, 31(2) 560-5; H. A. Lubs TheChemistry of Synthetic Dyes and Pigments; Hafner; New York, N.Y.; 1955Chapter 5; in H. Zollinger Color Chemistry: Synthesis, Properties andApplications of Organic Dyes and Pigments; VCH; New York, N.Y., pp.67-73, 1987, and in U.S. Pat. No. 5,149,807; and EPO Laid OpenApplication No. 0,244,399.

Further, as other image-forming materials, materials where the mobilityof the compound having a dye part changes as a result of anoxidation-reduction reaction with silver halide, or an organic silversalt at high temperature can be used, as described in Japanese PatentApplication No. 165054 (1984). Many of the above-described materials arematerials wherein an image-wise distribution of mobile dyescorresponding to exposure is formed in the photosensitive material byheat development. Processes of obtaining visible images by transferringthe dyes of the image to a dye-fixing material (diffusion transfer) havebeen described in the above-described cited patents and Japanese PatentApplication Nos. 168,439 (1984) and 182,447 (1984).

Still further the reducing agent may be a compound that releases aconventional photographic dye coupler or developer upon oxidation as isknown in the art. When the heat developable, photosensitive element usedin this invention is heat developed in a substantially water-freecondition after or simultaneously with imagewise exposure, a mobile dyeimage is obtained simultaneously with the formation of a silver imageeither in exposed areas or in unexposed areas with exposedphotosensitive silver halide.

The total amount of optional leuco dye used as a reducing agent utilizedin the present invention should preferably be in the range of 0.5-25weight percent, and more preferably, in the range of 1-10 weightpercent, based upon the total weight of each individual layer in whichthe reducing agent is employed.

The Binder

The photosensitive silver halide (when used), the non-photosensitivereducible source of silver (i.e., the silver-carboxylate/1,2-diazinecoordination compound), the reducing agent, the optional leuco dye, andother addenda used in the present invention are generally added to atleast one binder as described herein below.

The binder(s) that can be used in the present if invention can beemployed individually or in combination with one another. It ispreferred that the binder be selected from polymeric materials, such as,for example, natural and synthetic resins and that the binder besufficiently polar to hold the other ingredients of the emulsion insolution or suspension. The binder may be hydrophilic or hydrophobic.

A typical hydrophilic binder is a transparent or translucent hydrophiliccolloid, examples of which include a natural substance, for example, aprotein such as gelatin, a gelatin derivative, a cellulose derivative,etc.; a polysaccharide such as starch, gum arabic, pullulan, dextrin,etc.; and a synthetic polymer, for example, a water-soluble polyvinylcompound such as polyvinyl alcohol, polyvinyl pyrrolidone, acrylamidepolymer, etc. Another example of a hydrophilic binder is a dispersedvinyl compound in latex form which is used for the purpose of increasingdimensional stability of a photographic element.

Examples of typical hydrophobic binders are polyvinyl acetals, polyvinylchloride, polyvinyl acetate, cellulose acetate, polyolefins, polyesters,polystyrene, polyacrylonitrile, polycarbonates, methacrylate copolymers,maleic anhydride ester copolymers, butadiene-styrene copolymers, and thelike. Copolymers, e.g. terpolymers, are also included in the definitionof polymers. The polyvinyl acetals, such as polyvinyl butyral andpolyvinyl formal, and vinyl copolymers such as polyvinyl acetate andpolyvinyl chloride are particularly preferred. The binders can be usedindividually or in combination with one another. Although the binder maybe hydrophilic or hydrophobic, it is preferably hydrophobic.

The binders are generally used at a level of from about 20 to about 80%by weight of the emulsion layer, and preferably, from about 30 to about55% by weight. Where the proportions and activities of leuco dyesrequire a particular developing time and temperature, the binder shouldbe able to withstand those conditions. Generally, it is preferred thatthe binder not decompose or lose its structural integrity at 200° F.(90° C.) for 30 seconds, and more preferred that it not decompose orlose its structural integrity at 300° F. (149° C.) for 30 seconds.

Optionally, these polymers may be used in combination of two or morethereof. Such a polymer is used in an amount sufficient to carry thecomponents dispersed therein, that is, within the effective range of theaction as the binder. The effective range can be appropriatelydetermined by one skilled in the art.

Photothermographic and Thermographic Formulations

The formulation for the photothermographic and thermographic emulsionlayer can be prepared by dissolving and dispersing the binder, thephotosensitive silver halide (when used), the non-photosensitivereducible source of silver; comprising a silver-carboxylate/1,2-diazinecoordination compound; the reducing agent for the non-photosensitivereducible silver source (as, for example, the optional leuco dye), andoptional additives, in an inert organic solvent, such as, for example,toluene, 2-butanone, or tetrahydrofuran.

The use of "toners" or derivatives thereof which improve the image, ishighly desirable, but is not essential to the element. Toners may bepresent in amounts of from 0.01 to 10 percent by weight of the emulsionlayer, preferably from 0.1 to 10 percent by weight. Toners are wellknown materials in the photothermographic art as shown in U.S. Pat. Nos.3,080,254; 3,847,612; and 4,123,282.

Examples of toners include phthalimide and N-hydroxyphthalimide; cycliciraides such as succinimide, pyrazoline-5-ones, and a quinazolinone,1-phenylurazole, 3-phenyl-2-pyrazoline-5-one, quinazoline and2,4-thiazolidinedione; naphthalimides such asN-hydroxy-1,8-naphthalimide; cobalt complexes such as cobaltic hexaminetrifluoroacetate; mercaptans as illustrated by 3-mercapto-1,2,4-triazole, 2,4-dimercaptopyrimidine,3-mercapto-4,5-diphenyl-1,2,4-triazole and2,5-dimercapto-1,3,4-thiadiazole; N-(aminomethyl)aryldicarboximides,e.g. (N,N-dimethylaminomethyl)phthalimide, andN-(dimethylaminomethyl)naphthalene-2,3-dicarboximide, and a combinationof blocked pyrazoles, isothiuronium derivatives and certain photobleachagents, e.g., a combination ofN,N'-hexamethylene-bis(1-carbamoyl-3,5-dimethylpyrazole),1,8-(3,6-diaza-octane)bis(isothiuronium)trifluoroacetate and2-(tribromomethylsulfonyl benzothiazole); and merocyanine dyes such as3-ethyl-5-[(3-ethyl-2-benzothiazolinylidene)-1-methyl-ethylidene]-2-thio-2,4-o-azolidinedione;phthalazinone, phthalazinone derivatives or metal salts or thesederivatives such as 4-(1-naphthyl)phthalazinone, 6-chlorophthalazinone,5,7-dimethoxyphthalazinone, and 2,3-dihydro-1,4-phthalazinedione; acombination of phthalazine plus one or more phthalic acid derivatives,e.g., phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid, andtetrachlorophthalic arthydride; quinazolinediones, benzoxazine ornaphthoxazine derivatives; rhodium complexes .functioning not only astone modifiers but also as sources of halide ion for silver halideformation in situ, such as ammonium hexachlororhodate (III), rhodiumbromide, rhodium nitrate and potassium hexachlororhodate (III);inorganic peroxides and persulfates, e.g., ammonium peroxydisulfate andhydrogen peroxide; benzoxazine-2,4-diones such as1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione, and6-nitro-1,3-benzoxazine-2,4-dione; pyrimidines and asym-triazines, e.g.,2,4-dihydroxypyrimidine, 2-hydroxy-4 -aminopyrimidine, and azauracil,and tetrazapentalene derivatives, e.g.,3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetrazapentalene, and1,4-di(o-chlorophenyl)-3,6-dimercapto-1H,4H-2,3a,5,6a-tetrazapentalene.

When used in photothermographic elements, the photothermographicelements used in this invention may be further protected against theadditional production of fog and can be stabilized against loss ofsensitivity during keeping. While not necessary for the practice Of theinvention, it may be advantageous to add mercury (II) salts to theemulsion layer(s) as an antifoggant. Preferred mercury (II) salts forthis purpose are mercuric acetate and mereuric bromide.

Suitable antifoggants and stabilizers, which can be used alone or incombination, include the thiazolium salts described in U.S. Pat. No.2,131,038 and U.S. Pat. No. 2,694,716; the azaindenes described in U.S.Pat. Nos. 2,886,437 and 2,444,605; the mercury salts described in U.S.Pat. No. 2,728,663; the urazoles described in U.S. Pat. No. 3,287,135;the sulfocatechols described in U.S. Pat. No. 3,235,652; the oximesdescribed in British Patent No. 623,448; the polyvalent metal saltsdescribed in U.S. Pat. No. 2,839,405; the thiuronium salts described inU.S. Pat. No. 3,220,839; and palladium, platinum and gold saltsdescribed in U.S. Pat. Nos. 2,566,263 and 2,597,915.

Photothermographic and thermographic elements of the invention maycontain plasticizers and lubricants such as polyalcohols, e.g., glycerinand diols of the type described in U.S. Pat. No. 2,960,404; fatty acidsor esters such as those described in U.S. Pat. Nos. 2,588,765 and3,121,060; and silicone resins such as those described in British PatentNo. 955,061.

The photothermographic and thermographic elements of the presentinvention may include image dye stabilizers. Such image dye stabilizersare illustrated by U.K. Patent No. 1,326,889; and U.S. Pat. Nos.3,432,300; 3,698,909; 3,574,627; 3,573,050; 3,764,337; and 4,042,394.

Photothermographic elements containing emulsion layers according to thepresent invention can be used in photographic elements which containlight-absorbing materials and filter dyes such as those described inU.S. Pat. Nos. 3,253,921; 2,274,782; 2,527,583; and 2,956,879. Ifdesired, the dyes can be mordanted, for example, as described in U.S.Pat. No. 3,282,699.

Photothermographic and thermographic elements containing emulsion layersdescribed herein may contain matting agents such as starch, titaniumdioxide, zinc oxide, silica, and polymeric beads including beads of thetype described in U.S. Pat. Nos. 2,992,101 and 2,701,245.

Emulsions in accordance with this invention may be used inphotothermographic and thermographic elements which contain antistaticor conducting layers, such as layers that comprise soluble salts, e.g.,chlorides, nitrates, etc., evaporated metal layers, ionic polymers suchas those described in Minsk, U.S. Pat. Nos. 2,861,056, and 3,206,312 orinsoluble inorganic salts such as those described in Trevoy, U.S. Pat.No. 3,428,451.

Photothermographic and Thermographic Formulations and Constructions

The photothermographic and thermographic elements of this invention maybe constructed of one or more layers on a substrate. Single layerconstructions should contain the silver halide (when used), thenon-reducible silver source material, the reducing agent, and binder aswell as optional materials such as toners, dye-forming materials,coating aids, and other adjuvants. Two-layer constructions shouldcontain silver halide (when used) and non-reducible silver source in oneemulsion layer (usually the layer adjacent to the substrate) and some ofthe other ingredients in the second layer or both layers, although twolayer constructions comprising a single emulsion layer coatingcontaining all the ingredients and a protective topcoat are envisioned.Multicolor photothermographie dry silver constructions may contain setsof these bilayers for each color or they may contain all ingredientswithin a single layer as described in U.S. Pat. No. 4,708,928. In thecase of multilayer, multicolor photothermographic elements, the variousemulsion layers are generally maintained distinct from each other by theuse of functional or non-functional barrier layers between the variousphotosensitive layers as described in U.S. Pat. No. 4,460,681.

Development conditions will vary, depending on the construction used,but will typically involve heating the imagewise exposed material at asuitably elevated temperature.

When used in a photothermographie element, the latent image obtainedafter exposure of the heat-sensitive construction can be developed byheating the material at a moderately elevated temperature of, forexample, about 80° C. to about 250° C., preferably from about 120° C. toabout 200° C., for a sufficient period of time, generally from 1 secondto 2 minutes. Heating may be carried out by the typical heating meanssuch as a hot plate, an iron, a hot roller, a heat generator usingcarbon or titanium white, or the like.

In some methods, the development is carried out in two steps. Thermaldevelopment takes place at a higher temperature, e.g. about 150° C. forabout 10 seconds, followed by thermal diffusion at a lower temperature,e.g. 80° C., in the presence of a transfer solvent. The second heatingstep at the lower temperature prevents further development and allowsthe dyes that are already formed to diffuse out of the emulsion layer tothe receptor layer.

When used in a thermographic element, the image may be developed merelyby heating at the above noted temperatures using a thermal stylus orprint head, or by heating while in contact with a heat absorbingmaterial.

Photothermographic and thermographic emulsions used in this inventioncan be coated by various coating procedures including wire wound rodcoating, dip coating, air knife coating, curtain coating, or extrusioncoating using hoppers of the type described in U.S. Pat. No. 2,681,294.If desired, two or more layers may be coated simultaneously by theprocedures described in U.S. Pat. No. 2,761,791 and British Patent No.837,095. Typical wet thickness of the emulsion layer can range fromabout 10 to about 100 micrometers (μm), and the layer can be dried inforced air at temperatures ranging from 20° C. to 100° C. It ispreferred that the thickness of the layer be selected to provide maximumimage densities greater than 0.2, and, more preferably, in the range 0.5to 2.5, as measured by a MacBeth Color Densitometer Model TD 504 usingthe color filter complementary to the dye color.

Additionally, it may be desirable in some instances to coat differentemulsion layers on both sides of a transparent substrate, especiallywhen it is desirable to isolate the imaging chemistries of the differentemulsion layers.

Barrier layers, preferably comprising a polymeric material, may also bepresent in the photothermographic element of the present invention.Polymers for the material of the barrier layer can be selected fromnatural and synthetic polymers such as gelatin, polyvinyl alcohols,polyacrylic acids, sulfonated polystyrene, and the like. The polymerscan optionally be blended with barrier aids such as silica.

Alternatively, the formulation may be spray-dried or encapsulated toproduce solid particles, which can then be redispersed in a second,possibly different, binder and then coated onto the support.

The formulation for the emulsion layer can also include coating aidssuch as fluoroaliphatic polyesters.

Photothermographic emulsions used in the invention can be coated on awide variety of supports. The support or substrate can be selected froma wide range of materials depending on the imaging requirement.Substrates may be transparent or opaque. Typical supports includepolyester film, subbed polyester film, polyethylene terephthalate film,cellulose nitrate film, cellulose ester film, polyvinyl acetal film,polycarbonate film and related or resinous materials, as well as glass,paper, metal and the like. Typically, a flexible support is employed,especially a paper support, which can be partially acetylated or coatedwith baryta and/or an α-olefin polymer, particularly a polymer of analpha-olefin containing 2 to 10 carbon atoms such as polyethylene,polypropylene, ethylene-butene copolymers, and the like. Preferredpolymeric materials for the support include polymers having good heatstability, such as polyesters. A particularly preferred polyester ispoly(ethylene terephthalate).

The substrate with backside resistive heating layer may also be used incolor photothermographic imaging systems such as shown in U.S. Pat. Nos.4,460,681 and 4,374,921.

The Image-Receiving Layer

When the reactants and reaction products of photothermographic andthermographic systems that contain compounds capable of being oxidizedto form or release a dye remain in contact after imaging, severalproblems can result. For example, thermal development often forms turbidand hazy color images because of dye contamination by the reducedmetallic silver image on the exposed area of the emulsion. In addition,the resulting prints tend to develop color in unimaged background areas.This "background stain" is caused by slow reaction between thedye-forming or dye-releasing compound and reducing agent during storage.It is therefore desirable to transfer the dye formed upon imaging to areceptor, or image-receiving layer.

Thus, the photothermographic or thermographic element may furthercomprise an image-receiving layer. Images derived from thephotothermographic elements employing compounds capable of beingoxidized to form or release a dye, such as, as for example, leuco dyes,are typically transferred to an image-receiving layer.

If used, dyes generated during thermal development of light-exposedregions of the emulsion layers migrate under development conditions intothe an image-receiving or dye-receiving layer wherein they are retained.The dye-receiving layer may be composed of a polymeric material havingaffinity for the dyes employed. Necessarily, it will vary depending onthe ionic or neutral characteristics of the dyes.

The image-receiving layer of this invention can be any flexible orrigid, transparent layer made of thermoplastic polymer. Theimage-receiving layer preferably has a thickness of at least 0.1 μm morepreferably from about 1 to about 10 μm, and a glass transitiontemperature (T_(g)) of from about 20° C. to about 200° C. In the presentinvention, any thermoplastic polymer or combination of polymers can beused, provided the polymer is capable of absorbing and fixing the dye.Because the polymer acts as a dye mordant, no additional fixing agentsare required. Thermoplastic polymers that can be used to prepare theimage-receiving layer include polyesters, such as polyethyleneterephthalates, polyolefins, such as polyethylene, cellulosics, such ascellulose acetate, cellulose butyrate, cellulose propionate;polystyrene; polyvinyl chloride; polyvinylidine chloride, polyvinylacetate; copolymer of vinyl chloride-vinyl acetate, copolymer ofvinylidene chloride-acrylonitrile; copolymer of styrene-acrylonitrile,and the like.

The optical density of the dye image and even the actual color of thedye image in the image-receiving layer is very much dependent on thecharacteristics of the polymer of the image-receiving layer, which actsas a dye mordant, and, as such, is capable of absorbing and fixing thedyes. A dye image having a reflection optical density in the range offrom 0.3 to 3.5 (preferably, from 1.5 to 3.5) or a transmission opticaldensity in the range of from 0.2 to 2.5 (preferably, from 1.0 to 2.5) isdesirable.

The image-receiving layer can be formed by dissolving at least onethermoplastic polymer in an organic solvent (e.g., 2-butanone, acetone,tetrahydrofuran) and applying the resulting solution to a support baseor substrate by various coating methods known in the art, such ascurtain coating, extrusion coating, dip coating, air-knife coating,hopper coating, and any other coating method used for coating solutions.After the solution is coated, the image-receiving layer is dried (e.g.,in an oven) to drive off the solvent. The image-receiving layer may bestrippably adhered to the photothermographic element. Strippableimage-receiving layers are described in U.S. Pat. No. 4,594,307,incorporated herein by reference.

Selection of the binder and solvent to be used in preparing the emulsionlayer significantly affects the strippability of the image-receivinglayer from the photosensitive element. Preferably, the binder for theimage-receiving layer is impermeable to the solvent used for coating theemulsion layer and is incompatible with the binder used for the emulsionlayer. The selection of the preferred binders and solvents results inweak adhesion between the emulsion layer and the image-receiving layerand promotes good strippability of the emulsion layer.

The photothermographic element can also include coating additives toimprove the strippability of the emulsion layer. For example,fluoroaliphatic polyesters dissolved in ethyl acetate can be added in anamount of from about 0.02 to about 0.5 weight percent of the emulsionlayer, preferably from about 0.1 to about 0.3 weight percent. Arepresentative example of such a fluoroaliphatic polyester is "FluoradFC 431", (a fluorinated surfactant available from 3M Company, St. Paul,Minn.). Alternatively, a coating additive can be added to theimage-receiving layer in the same weight range to enhance strippability.No solvents need to be used in the stripping process. The strippablelayer preferably has a delaminating resistance of 1 to 50 g/cm and atensile strength at break greater than, preferably at least two timesgreater than, its delaminating resistance.

Preferably, the image-receiving layer is adjacent to the emulsion layerin order to facilitate transfer of the dye that forms after theimagewise, exposed emulsion layer is subjected to thermal development,for example, in a heated shoe-and-roller-type heat processor.

Photothermographic multi-layer constructions containing blue-sensitiveemulsions containing a yellow leuco dye may be overcoated withgreen-sensitive emulsions containing a magenta leuco dye. These layersmay in turn be overcoated with a red-sensitive emulsion layer containinga cyan leuco dye. Imaging and heating form the yellow, magenta, and cyanimages in an imagewise fashion. The dyes so formed may migrate to animage-receiving layer. The image-receiving layer may be a permanent partof the construction or may be removable, "i.e., strippably adhered," andsubsequently peeled from the construction. Color-forming layers may bemaintained distinct from each other by the use of functional ornon-functional barrier layers between the various photosensitive layersas described in U.S. Pat. No. 4,460,681. False color address, such asthat shown in U.S. Pat. No. 4,619,892, may also be used rather thanblue-yellow, green-magenta, or red-cyan relationships betweensensitivity and dye formation.

In another embodiment, the colored dye released in the emulsion layercan be transferred onto a separately coated image-receiving sheet byplacing the exposed emulsion layer in intimate face-to-face contact withthe image-receiving sheet and heating the resulting compositeconstruction. Good results can be achieved in this second embodimentwhen the layers are in uniform contact for a period of time of from 0.5to 300 seconds at a temperature:of from about 80° C. to about 220° C.

Alternatively, a multi-colored image may be prepared by superimposing inregister a single image-receiving sheet successively with two or moreimagewise exposed photothermographic or thermographic elements, each ofwhich release a dye of a different color, and heating to transfer thereleased dyes .as described above. This method is particularly suitablefor the production of color proofs especially when the dyes releasedhave hues which match the internationally-agreed standards for colorreproduction (SWOP colors). Dyes with this property are disclosed inU.S. Pat. No. 5,023,229. In this embodiment, the photothermographic orthermographic element preferably comprise compounds capable of beingoxidized to release a pre-formed dye as this enables the image dyeabsorptions to be tailored more easily to particular requirements of theimaging system. When used in a photothermographic element, the elementsare preferably all sensitized to the same wavelength range regardless ofthe color of the dye released. For example, the elements may besensitized to ultra-violet radiation with a view toward contact exposureon conventional printing frames, or they may be sensitized to langerwavelengths, especially red or near infra-red to enable digital addressby lasers.

Objects and advantages of this invention will now be illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention. All percentagesare by weight unless otherwise indicated.

EXAMPLES

All materials used in the following examples were readily available fromstandard commercial sources such as Aldrich Chemical Co. (Milwaukee,Wis.) unless otherwise specified. The following additional terms andmaterials were used.

Butvar™ B-72 is a poly(vinyl butyral) available from Monsanto Company,St. Louis, Mo.

PET is poly(ethylene terephthalate).

CAO-5 is an antioxidant available from Rohm and Haas, Philadelphia, Pa.and has the structure shown below: ##STR5##

EXAMPLE 1 Preparation of Coordination Compounds

Preparation of [Ag(PHZ)(O₂ CCH₃)(H₂ O)₂ ]₂ : Mixing stoichiometricamounts of PHZ and silver acetate under ambient conditions (roomtemperature and subdued laboratory light) in water yielded colorless,air and light stable crystals.

Preparation of other carboxylate derivatives [Ag(PHZ)(O₂ CR¹)]₂ : 1:1mole ratios of reactants were dissolved or dispersed in water, stirredfor one hour, filtered, washed and air dried in the dark. For compoundsof [Ag(PHZ)(O₂ CR¹)]₂ having water solubility, the filtrates wereallowed to slowly evaporate to yield highly crystalline solids. In allcases, elemental microanalysis confirmed the presence of the PHZ in thesilver-carboxylate coordination compound.

Evaluation of Coordination Compounds in a Black-and-White Construction

The silver-carboxylate/1,2-diazine compounds were milled in the dark(0.7 g in 7.0 g of 5% Butvar™ B-72 in ethanol), coated 3 mils wet, ovendried at 80° C. for 2 minutes and cut into strips. The strips werestreaked with a 20% solution of CAO-5 in ethanol and, air dried. Thesamples were placed on a thermal wedge (Reichort Hot Bench™) for 6seconds and immediately thermally quenched on a room temperature heatsink. The temperatures for the onset of thermal imaging and approximateD_(max) were measured. The results are shown in the table below anddemonstrate the constructions possible for the thermally activatedimaging properties of these novel silver coordination complexs. Otherphenolic developers in addition to CAO-5, such as hydroquinone, havebeen found to produce thermally generated images with these materials.

    ______________________________________                                        R.sup.1 in [Ag(PHZ)(O.sub.2 CR.sup.1)].sub.2                                                 T.sub.onset °C.                                                                 T.sub.Dmax °C.                                                                   Image Color                                 ______________________________________                                        --CH.sub.2 CH.sub.2 CO.sub.2 H                                                               100      120       brown-black                                 --CF.sub.3     110      130       tan                                         --CH.sub.2 NH.sub.2                                                                          RT                 dark                                                                          red-brown                                   --C(CH.sub.3).sub.3                                                                          RT                 dark brown                                  --(CH.sub.2).sub.4 CH.sub.3                                                                  RT                 black                                       --(CH.sub.2).sub.12 CH.sub.3                                                                 100      120       dark brown                                  --(CH.sub.2).sub.11 OH                                                                       100      120       dark brown                                  (4-CH.sub.3)C.sub.6 H.sub.3 CO.sub.2 AgPHZ                                                   RT                 purple-black                                --C.sub.6 H.sub.4 -ρ-CN                                                                  105      140       brown-black                                 --C.sub.6 H.sub.4 -ρ-C(CH.sub.3).sub.3                                                    90      135       dark brown                                  ______________________________________                                         RT = Room Temperature                                                    

EXAMPLE 2 Evaluation of Coordination Compounds in a Color-imagingConstruction

The silver-carboxylate/1,2-diazine compounds were milled in the dark(0.7 g in 7.0 g of 5% Butvar™ B-72 in ethanol), coated 3 mils wet, ovendried at 80° C. for 2 minutes and cut into strips. The strips werestreaked with a 2% solution of the leuco dye in ethanol ortetrahydrofuran and air dried. The samples were placed on a thermalwedge (Reichert Hot Bench™) for 6 seconds and immediately thermallyquenched on a heat sink (at room temperature). The temperatures for theonset of thermal imaging and approximate D_(max) were measured. Theresults are shown in the attached table and demonstrate the simpleconstructions possible for the thermally activated imaging properties ofthese novel silver coordination compounds for thermographic colorimaging.

The leuco dyes tested have the following structures:

Leuco Dye-1 is an leuco oxazine dye described in U.S. Pat. No. 4,782,010and has the formula shown below.

Leuco Dye-2 is a leuco benzimidazole dye and is described in U.S. Pat.No. 3,985,565.

Leuco Dye-3 is a bis-phenol leuco dye and is described in U.S. Pat. No.4,535,056.

Leuco Dye-4 is a leuco ketazine dye described in U.S. Pat. Nos.4,587,211 and 4,795,697 and has the formula shown below.

    ______________________________________                                         ##STR6##                                                                      ##STR7##                                                                      ##STR8##                                                                      ##STR9##                                                                     R.sup.1 in              T.sub.onset                                                                          T.sub.Dmax                                                                          Image                                    [Ag(PHZ)(O.sub.2 CR.sup.1)].sub.2                                                          Dye        °C.                                                                           °C.                                                                          Color                                    ______________________________________                                        (CH.sub.2).sub.11 OH                                                                       Leuco Dye-1                                                                              115    135   blue-green                               CH.sub.2 CH.sub.2 CO.sub.2 H                                                               "           89    130   "                                        C.sub.6 H.sub.4 -p-CN                                                                      "          114    133   "                                        C.sub.6 H.sub.4 -p-t-butyl                                                                 "          110    130   "                                        (CH.sub.2).sub.4 CH.sub.3                                                                  "          RT           "                                        (CH.sub.2).sub.11 OH                                                                       Leuco Dye-2                                                                              100    115   yellow                                   CH.sub.2 CH.sub.2 CO.sub.2 H                                                               "          110    121   "                                        C.sub.6 H.sub.4 -p-CN                                                                      "          120    140   "                                        C.sub.6 H.sub.4 -p-t-butyl                                                                 "          120    140   "                                        (CH.sub.2).sub.4 CH.sub.3                                                                  "          115    135   "                                        (CH.sub.2).sub.11 OH                                                                       Leuco Dye-4                                                                              100    135   magenta                                  CH.sub.2 CH.sub.2 CO.sub.2 H                                                               "          100    115   "                                        C.sub.6 H.sub.4 -p-CN                                                                      "          110    127   "                                        C.sub.6 H.sub.4 -p-t-butyl                                                                 "          120    >150  "                                        (CH.sub.2).sub.4 CH.sub.3                                                                  "           90    119   "                                        (CH.sub.2).sub.11 OH                                                                       Leuco Dye-3                                                                               88    110   yellow                                   CH.sub.2 CH.sub.2 CO.sub.2 H                                                               "           90    124   "                                        C.sub.6 H.sub.4 -p-CN                                                                      "          120    132   "                                        C.sub.6 H.sub.4 -p-t-butyl                                                                 "          100    130   "                                        (CH.sub.2).sub.4 CH.sub.3                                                                  "           80    100   "                                        ______________________________________                                         RT = Room Temperature                                                    

Reasonable modifications and variations are possible from the foregoingdisclosure without departing from either the spirit or scope of thepresent invention as defined by the claims.

What is claimed is:
 1. A photothermographic element comprising a supportbearing at least one heat-developable, photosensitive, image-formingphotothermographic emulsion layer comprising:(a) a photosensitive silverhalide; (b) a non-photosensitive, reducible source of silver comprisinga silver-carboxylate/1,2-diazine coordination compound of the formula:##STR10## wherein: R¹ is either an alkyl, aralkyl, cycloalkyl, andalkenyl group of up to 29 carbon atoms; or an aryl group of up to 14carbon atoms; and R² represents either hydrogen, an alkyl group, acycloalkyl group fused to the 1,2-diazine ring, or the atoms necessaryto complete a 5- or 6-membered aromatic ring fused to the 1,2-diazinering; (c) a reducing agent for said non-photosensitive, reducible sourceof silver; and (d) a binder.
 2. The photothermographic element accordingto claim 1 wherein R¹ is either an alkyl, aralkyl, cycloalkyl, oralkenyl group of from 9 to 29 carbon atoms or an aryl group of up to 10carbon atoms.
 3. The photothermographic element according to claim 2wherein R¹ is either an alkyl, aralkyl, cycloalkyl, or alkenyl group offrom 14 to 27 carbon atoms.
 4. The photothermographic element accordingto claim 1 wherein R² represents the atoms necessary to complete a 5- or6-membered aromatic ring fused to a 1,2-diazine ring.
 5. Thephotothermographic element of claim 1 wherein said reducing agent forsaid non-photosensitive reducible source of silver comprises a compoundcapable of being oxidized to form or release a dye.
 6. Thephotothermographic element of claim 5 wherein said compound capable ofbeing oxidized to form or release a dye is a leuco dye.
 7. Athermographic element comprising a substrate coated with an imaginglayer comprising:(a) a non-photosensitive, reducible source of silvercomprising a silver-carboxylate/1,2-diazine compound of the formula:##STR11## wherein: R¹ is either an alkyl, aralkyl, cycloalkyl, oralkenyl group of up to 29 carbon atoms; or an aryl group of up to 14carbon atoms; R² represents either hydrogen, an alkyl group, acycloalkyl group fused to the 1,2-diazine ring, or the atoms necessaryto complete a 5- or 6-membered aromatic ring fused to the 1,2-diazinering; (b) a reducing agent for said non-photosensitive reducible sourceof silver; and (c) a binder.
 8. The thermographic element of claim 7wherein R¹ is either an alkyl, aralkyl, cycloalkyl, or alkenyl group offrom 9 to 29 carbon atoms or an aryl group of up to 10 carbon atoms. 9.The thermographic element of claim 8 wherein R¹ is either an alkyl,aralkyl, cycloalkyl, or alkenyl group of from 14 to 27 carbon atoms. 10.The thermographic element of claim 7 wherein R² represents the atomsnecessary to complete a 5- or 6-membered aromatic ring fused to a1,2-diazine ring.
 11. The thermographic element of claim 7 wherein saidreducing agent for said non-photosensitive reducible source of silvercomprises a compound capable of being oxidized to form or release a dye.